With the constant increase of mobile data services and emergence of new-type applications, the 3rd Generation Partnership Project (3GPP) organization has developed long-term evolution (LTE) specifications and LTE-Advanced (LTE-A) specifications. As the next generation cellular communication standard, an LTE or LTE-Advance system can operate in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.
To meet constantly increasing requirements on data rate and the spectrum efficiency, there was proposed Multiple Input Multiple Output (MIMO) technology to improve the network performance. The MIMO technology means the use of multiple antennas at both the transmitter and receiver to improve communication performance. Recently, the MIMO technology has attracted much attention in wireless communications, because it could offer significant increases in both data throughput and link range without additional bandwidth or increased transmit power. Moreover, the MIMO technology has become a key feature of the LTE/LTE-A system.
Currently, it is known that one-dimensional antenna array can provide a flexible beam adaption in horizontal plane by a horizontal precoding process while a down-tilt is applied in a vertical domain. However, it has been recently found that a full MIMO capability could also be exploited through leveraging a two-dimensional antenna planar such that user-specific beamforming and spatial multiplexing on the vertical domain is possible.
To help improving transmit and/or receive gains and reducing interference, it has proposed to study user-specific beamforming and full dimension MIMO (i.e., 3D MIMO) with 2D antenna array (Active Antenna System, AAS).
However, the UE-specific beamforming and spatial multiplexing have not been provided in elevation domain yet, i.e. there is no vertical precoding process yet. The 3D MIMO can solve this problem with vertical precoding process. It may provide a flexible beam adaption both for a horizontal domain and a vertical domain by horizontal and vertical precoding process, respectively.
In WO2013/024351 is disclosed a method of designing codebook for 3D antenna configuration. The disclosed solution mainly relates to codebook design, and in this discourse, there are designed two codebooks, i.e., a horizontal codebook and a vertical codebook, which correspond to the horizontal domain and the vertical domain respectively. For each row/column in the antenna array, the same codeword could be used to quantize corresponding horizontal/vertical channel state information. A user equipment (UE) feeds back Precoding Matrix Indicators (PMIs) from the horizontal and vertical codebooks respectively and at an evolved node B (eNB), the 3D precoding matrix will be composed from the two PMIs.
In a Chinese patent application publication No. CN102938688A, there is disclosed a solution for channel measurement and feedback of a multi-dimensional antenna array, in which an eNB will send two types of reference signals corresponding to a horizontal reference signal and a vertical reference signal in the same subframe or in different subframes and the UE receives the two types of reference signals from eNB and measures and feeds back two classes CSI corresponding horizontal CSI and vertical CSI to the eNB.
Besides, in 3GPP document R1-112420 entitled “Considerations on CSI feedback enhancements for high-priority antenna configurations”, there is specified how to form 3D precoding matrix by horizontal precoding matrix and vertical precoding matrix. For a purpose of illustration, FIG. 1 illustrates a schematic diagram of the solution as disclosed in the documents. As illustrated in FIG. 1, the vertical CSI-RS and horizontal CSI-RS are transmitted to the UE in subframe 1 and two 4Tx PMIs selection and feedback for horizontal CSI and vertical CSI will be received from the UE respectively. Then, the eNB may reconstruct the CSI based on horizontal and vertical PMIs by means of extrapolation.